Ferromagnetic materials



United States Patent FERROMAGNETIC MATERIALS Thomas R. McGuire andMerrill W. Shafer, Yorktown Heights, N.Y., assignors to InternationalBusiness Machines Corporation, New York, N.Y., a corporation of New YorkNo Drawing. Filed Jan. 28, 1965, Ser. No. 428,862

8 Claims. (Cl. 252 62.51)

ABSTRACT OF THE DISCLQSURE Ferromagnetic compositions represented by theformula (EuO) (RS) in which 0.01 x 0.30 and R is a rare earth. Thecompositions are characterized by a Curie temperature which can becontrolled between 73 K. and 132 K.

This invention relates to a new series of europium compounds exhibitinga paramagnetic Curie temperature (0) which can be selectively variedbetween 73 K. and 132 K. In accordance with this invention, the paramagnetic Curie temperature (0) of such compounds is varied by changingthe population of electrons in the d-orbitals of the base europium oxide(EuO) material while retaining the characteristically high magneticmoment thereof.

With the discovery of ferromagnetism in the europium chalcogenideseries, a new group of materials with device applications has becomeavailable. One such material is europium oxide (EuO) which exhibitsdesirable characteristics, i.e., high magnetic moment; however, europiumoxide (EuO) exhibits a relatively low paramagnetic Curie temperature (0)of 73 K. Paramagnetic Curie temperature (0) is a measure of theferromagnetic exchange interactions and is closely related to the Curietemperature where spontaneous magnetization occurs. Accordingly, presentuse of europium oxide for device applications is seriously limited sincedevices formed thereof cannot be operated at convenient temperatures,say liquid nitrogen (N temperatures (77 K.). The usefulness of europiumoxide (EuO) would beobviously increased if the respective paramagneticCurie temperature (0) exhibited thereby could be raised at least abovethe temperature of liquid nitrogen (N at atmospheric pressures.

The ferromagnetic saturation moment of europium oxide (EuO) is higherthan those of ferrites which, for example, have saturation moments below6,000 gauss. Therefore, europium oxide (EuO) would provide a usefulmagnetic material for application where high magnetic moments aredesired (high frequency transformer cores, microwave gyrators, etc.)

Generally, the structure of europium oxide (EuO) can be characterized ashaving a half-filled 4f shell and an empty 5d shell. Paramagnetic Curietemperature (0) is affected by changing the population of electrons inthe europium oxide (EuO) to vary electron concentrations in the crystallattice. According to molecular field theory, magnetic interactionsbetween electron spin moments is determinative of the paramagnetic Curietemperature (0). The interaction between magnetic spin moments whichproduces spontaneous magnetization in europium oxide (EuO) has beenexplained by the interaction between spin moments of the nearestneighbor europium (Eu) ions in the rock salt-type face centered cubiccrystal structure of europium oxide (EuO). Reference is made to patentapplication Ser. No. 374,351, filed June 11, 1964, in the names of F.Holtzberg et al., entitled New Ferromagnetic Materials, and assigned tothe assignee of this invention. Since the electron spin moments of the4f and 5d shells of neighboring europium ions (Eu++) 3,371 ,042 PatentedFeb. 27, 1968 overlap, there is a virtual occupation of the 5d shellwhich gives rise to exchange interactions. The virtual occupation of theS'd shell would allow the filling of the 1' orbitals which would overlapin the direction and effectively increase magnetic interactions betweenneighboring europium ions (Eu++) whereby the para-magnetic Curietemperature (9) is increased. The amount of increase in paramagneticCurie temperature (0) varies as the increased electron concentration inthe crystal lattice of the europium oxide (EuO).

In accordance with this invention, the paramagnetic Curie temperature(6) of europium oxide (EuO) is ad justed by forming a solid solutionhaving the formula (EuO) (RS) where 0.0l x 0.30, (RS) is a monosulfide,and (R) is a rare earth selected, from the group consisting of lanthanum(La), praseodymium (Pr), neodymium (Nd), saznarium (SM), gadolinium(Gd), cerium (Ce), dysprosium (Dy), terbium (Tb), holmium (Ho), erbium(Er), thulium (Tm), and lutetium (Lu). In accordance with thisinvention, albeit a two-phase system may result, the solid solution ofthe monosulfide (RS) in europium oxide (EuO) exhibits a substantialincrease in paramagnetic temperature (0).

It is an object of this invention to provide a new series of europiumcompounds exhibiting a Curie temperature (0) which can be controlledbetween 73 K. and 132 K.

Another object of this invention is to prepare a material exhibiting arelatively high magnetic moment and having a paramagnetic Curietemperature (0) in excess of liquid nitrogen (N temperatures (77 K.).

Another object of this invention is to provide a process for controllingthe Curie temperature (6) of magnetic materials formed of europium oxide(EuO).

Another object of this invention is to provide a ferromagnetic materialhaving the formula (EuO) (RS) where 0.0l x 0.30.

The novel ferromagnetic material of this invention is prepared byhomogeneously mixing appropriate predetermined quantities of the baseeuropium oxide (EuO) and monosulfide material (RS), where (RS) is amonosulfide and (R) is a rare earth selected from the group consistingof lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm),gadolinium (Gd), cerium (Ce), dysprosium (Dy), terbium (Tb), holmium(H0), erbium (Er), thulium (Tm), and lutetium (Lu). The europium oxide(EuO) and monosulfide (RS), thus mixed, are heated to a reactiontemperature at least lower than the decomposition temperature of eithermaterial and then cooled rapidly to room temperature. The quantity ofmonosulfide material (RS) added to the base europium oxide (EuO)material is determinative of the change in paramagnetic Curietemperature (0) of the reacted mixture, i.e., of the europium oxide(EuO). The reacted mixture may define at least a two-phase system, onesuch phase being a solid solution monosulfide of the (RS) and europiumoxide (EuO).

This procedure involved in forming the novel ferromagnetic material ofthis invention is to initially blend, e.g., by mechanical shaker, mortarand pestle, etc., appropriate amounts of the base europium oxide (E110)material and the modifying monosulfide material (RS), each in finelydivided form, to obtain a homogeneous mixture, The homogeneous mixtureof the base europium oxide (EuO) material and the monosulfide material(RS) are pressed into pellets, e.g., by a hydraulic press, and thenheated to between l500 ing atmosphere. For example, the pellets may beheated in a refractory metal crucible, e.g., molybdenum, tantalum, etc.,which is evacuated and sealed. Alternatively, the mixture can be placedin an open crucible and heated in a reducing atmosphere, e.g., helium,argon, etc/In either event, the pellets are heated in the describedtemperature" C. and 1950 C. in a nonoxidiz-" range in excess of one hourso as to insure reaction of the base europium oxide (EuO) material andthe monosulfide material (RS) to support the formation of a solidsolution. Best results have been observed when the monosulfide(RS)-europium oxide (EuO) mixture is heated sufficiently to cause aportion of the mixture to pass into the liquid phase and rapidly cool toroom temperature, for example, within 30 minutes. The reacted mixture,when cooled, retains the desired magnetic properties of europium oxide(EuO) but exhibits a paramagnetic Curie temperature which can beadjusted to exceed liquid nitrogen (N temperatures.

The paramagnetic Curie temperature (6) is increased due to theintroduction of additional electrons into the rock-salt structure of thebase europium oxide (EuO) material by the monosulfide (RS) whereby theelectron concentration of the former is increased. Since the (R)constituent, above identified, of the monosulfide (RS) material istrivalent, it can be theorized that the substitution of trivalent (R+++)ions for the (Eu++) ions in the lattice introduced electrons into the dshell of remaining (Eu++) ions in the lattice. Due to the presence ofthe trivalent (R+++) ions, the 5d shells of the remaining (Eu++) ionsare virtually occupied and magnetically coupled in the (110) directionso as to increase magnetic interaction between adjacent (Eu++) ions inthe lattice. The resulting virtual occupation of the 5d shells (1 givesrise to exchange interactions eifective to increase the paramagneticCurie temperature (0) in excess of liquid nitrogen (N temperature (77K.).

Example 1 Ferromagnetic materials of this invention have been preparedin accordance with the following table where the following percentagesin moles of europium oxide (E110) and cerium sul ide (CeS) were reactedto obtain the indicated paramagnetic Curie temperature (0).

TABLE I Para-magnetic E110 (moles) CeS (moles) Curie temperature Theeuropium oxide (EuO) and cerium sulfide (CeS) materials in finelypowdered form in the amounts shown in Table I were mixed by mortar andpestle to obtain a homogeneous mixture. The homogeneous mixture waspressed into pellets and placed in an open tantalum crucible. .Thetantalum crucible was heated in an argon (Ar) atmosphere forapproximately 8 hours at 1700 C., a portion of the mixture beingobserved to pass into the liquid phase, and cooled down to roomtemperature within a 30-minute period. The reacted mixtures each formeda two-phase system and exhibited increased paramagnetic Curietemperature (6) which varied as the percentage by mole of the modifyingcerium sulfide (CeS) material, as shown in Table I. Part of the reactedmaterial had a rocksalt structure. When the percentage by mole of ceriumsulfide (CeS) was increased to 5 percent, the reacted mixture exhibtedferromagnetic properties at 78 K. or above liquid nitrogen (Ntemperatures. Also, the reacted mixture exhibited increased electricalconductivity as the percentage by mole of cerium sulfide (CeS) wasincreased. The observed paramagnetic Curie temperature (0) of 130 K.when 70 percent by mole of europium oxide (EuO) was reacted with 30percent by mole of cerium sulfide (CeS) was within experimental error.

Example 2 Also, similar procedures have been employed to prepareferromagnetic materials of this invention wherein the fol- 4 lowingpercentages by mole of europium oxide (EuO) and lanthanum sulfide .(LaS)as shown in the Table II' were reacted to obtain the indicatedparamagnetic Curie temperatures (0).

Finely-powdered europium oxide (EuO) and lanthanum sulfide (LaS)materials were homogeneously mixed by mortar and pestle and pressed intopellets. The pellets were placed into an open tantalum crucible andheated at 1750 C. in a nitrogen (N atmosphere for approximately 8 hoursand then cooled to room temperature within a 30- minute period. Duringthe heat treatment, a portion of the mixture was observed to pass intothe liquid phase. As the percentage by mole of lanthanum sulfide (LaS)was increased, the paramagnetic Curie temperature (0) of the reactedmixture rose as indicated in Table II. The presence of 5 percent by moleof lanthanum sulfide (LaS) was effective to increase the paramagneticCurie temperature (0) of the reacted mixture to approximately K., i.e.,in excess of liquid nitrogen (N temperatures. Also, the reacted mixtureseach formed a two-phase system and exhibited increased electricalconductivity as the PBICCHtage by mole of lanthanum sulfide (LaS) wasincreased.

Example 3 Similar procedures were followed as in Examples 1 and 2 withrespect to the europium oxide (EuO)-erbium sulfide (ErS) series. Fifteenpercent by mole of erbium sulfide (ErS) was reacted with percent by moleof europium oxide (EuO). Again, finely-powedered europium oxide (EuO)and erbium sulfide (ErS) were homogeneously mixed by mortar and pestle,and pressed into pellets. The pellets were sealed, however, in anevacuated tantalum crucible and heated at 1750 C. for approximately 8hours and cooled down to room temperature Within a 30 minute period. Thereacted mixture formed a two-phase system and exhibited a paramagneticCurie temperature (0) of Example 4 Similar procedures were followed asin Example" 3 with respect to the europium oxide (EuO)-gad-' oliniumsulfide (GdS) system. For example, finely-- powdered europium oxide(EuO) and gadolinium sulfide (GdS) were formed in a homogeneous mixtureof 85 percent-l5 percent by mole and pressed into pellets. The pelletswere ealed in an evacuated tantalum crucible and heated at 1750 C. forapproximately 8 hours. The reacted mixture was then cooled to roomtemperature within a 30 minute period. The reacted mixture formed atwophase system and exhibited a paramagnetic Curie temperature (0) of130 K.

Because of the similar chemical properties of the remaining reactants(RS), it is clear that similar solid solutions of europium oxide (EuO)and any of the (RS) monosulfides will give corresponding increases inparamagnetic Curie temperature (0).

While the invention has been particularly described with reference topreferred embodiments thereof, it will be understood by those skilled inthe art that various. changes in form and details may be made thereinwithout departing from the spirit and scope of the invention.

What is claimed is:

1. A ferromagnetic material having the formula 5 Where 0.0l x 0.30 and Ris a rare earth element selected from the group consisting of lanthanum(La), praseodymiurn (Pr), neodymium (Nd), samarium (Sm), gadolinium(Gd), cerium (Ce), dysprosium (Dy), terbium (Tb), holmium (Ho), erbium(Er), thulium (Tm), and lutetium (Lu).

2. A ferromagnetic material having the formula )1 x( )x where 0.01 x0.30.

3. A ferromagnetic material having the formula )1 x( )x Where 0.01 x0.30.

4. A ferromagnetic material having the formula )1 X( )x where 0.01 x0.30.

5. A ferromagnetic material having the formula where 0.01 x 0.30.

6. The method of forming a ferromagnetic material having the formula(EuO) (RS) where 0.01 x 0.30 and (RS) is a monosulfide of a rare earthelement selected from the group consisting of cerium (Ce), lanthanum(La), erbium (Br), and gadolinium (Gd), praseosaid mixture dymium (Pr),neodymium (Nd), samari-um (Sm), dysprosium (Dy), terbium (Tb), holmium(Ho), thulium (Tm), and lutetium (Lu) comprising the steps of mixingeuropium oxide (EuO) and said monosulfide, heating to react at least aportion of said mixture to form a solid solution of said europium oxide(EuO) and said monosulfide, and rapidly cooling said mixture to roomtemperature.

7. The method of claim 6 comprising the further step of cooling saidmixture to room temperature Within a 30 minute period.

8. The method of claim 6 comprising the further step of heating saidmixture whereby a portion thereof enters into a liquid phase.

References Cited UNITED STATES PATENTS 2/1966 Matthias 252-62.51

OTHER REFERENCES Didchenko et al.: Some Electrical and MagneticProperties of Rare Earth Monosu lfides and Nitrides, J. Phys. Chem.Solids, vol. 24, 1963, pages 863870.

TOBIAS E. LEVOW, Primary Examiner. ROBERT D. EDMONDS, Examiner.

